Field of the Invention
The present invention relates to a compound semiconductor device such as a high electron mobility transistor.
Background Art
Taking advantage of excellent high frequency characteristics thereof, high electron mobility transistors (HEMT) are used as amplifiers for receiving antennas for satellite broadcasting and vehicle-mounted millimeter wave radars or the like. Among HEMT epitaxial structures being commercialized in recent years, double-doped HEMTs are the mainstream, which have carrier supply layers on not only the upper side but also the lower side of a channel layer in order to supply carriers to the channel layer which is the carrier traveling layer at high concentration and efficiently.
As the carrier supply layer of a double-doped HEMT, a uniformly doped supply layer is used which has a desired thickness and in which an impurity is uniformly doped. In order to improve HEMT performance, a planar doped supply layer is also used which implements a high-concentration thin-film supply layer, that is, locally high concentration impurity doping.
However, locally high concentration impurity doping makes the impurity more likely to diffuse, and it is known that the impurity enters the channel layer from the planar doped supply layer on the lower side, preventing the HEMT performance from improving.
As such an impurity diffusion suppression method, reduction of thermal history through lowering the temperature and reducing the time period of epitaxial growth has been proposed (e.g., see Japanese Patent Application Laid-Open No. 7-249757). As an impurity diffusion suppression structure for a GaAs-based HEMT, a structure is proposed in which In is added to a GaAs or AlGaAs layer in the vicinity of a planar doped layer to reduce lattice distortion of the planar doped layer which is the cause of diffusion (e.g., see Japanese Patent Application Laid-Open No. 8-32052). There is also a proposal of a structure in which an Al mixed crystal layer is formed adjacent to the planar doped layer or the like to improve mobility.
In the technique of implementing temperature lowering of epitaxial growth, there is concern about deterioration of reliability because an impurity such as oxygen is more likely to be mixed in an Al containing layer in particular. On the other hand, in the technique of adding In to a layer in the vicinity of the planar doped layer, the epitaxial structure or growth process becomes complicated, making stable production difficult. Moreover, provision of the Al mixed crystal layer causes the Al mixed crystal layer to become a resistive layer and increase source series resistance, resulting in a problem that the high frequency characteristic deteriorates and mobility decreases.